In electronic systems in which one wants to increase the rejection to an external disturbance of the output signal an additional circuitry is generally used based on feed-forward techniques. The techniques act in such a manner to prevent the variation of the output value due to an external disturbance; the disturbance is measured and the corrective action is undertaken before the disturbance has spread to the output signal thereby increasing the immunity of the output signal to the external disturbance.
Electronic systems in which the feed-forward techniques are used are, for example, the control systems of the voice coil motor (VCM) of a hard disk based on a power stage whose output voltage is a function of the supply voltage, or also a driving system of a spindle motor of a hard disk.
A system type in which the output signal is a function of the supply voltage is the Pulse Width Modulation (PWM) driving system. In a PWM driving system the power stage only works in saturation state (that is towards the supply voltage or towards a reference voltage) and cut-off state. The percentage of the period of time in which the power stage supplies in output the voltage for supplying the load in relation to the total period of time of excitation is called duty-cycle. In a PWM driving system in which a compensation of the variations of the supply voltage does not exist the output signal is proportional to the input signal and to the noise.
Devices for compensating the variations of the supply voltage in driving systems have been used; in the digital driving systems the devices generally comprise the use of an analog-digital converter (ADC), sampled at a certain frequency, for reading the entity of the disturbance.
To keep the voltage constant at the terminals of the load in presence of disturbances or variations of the supply voltage various techniques are used that tend to make the product of the supply voltage for the duty-cycle independent from the variations. For this reason the duty-cycle is varied in function of the value of the supply voltage.
A method for keeping the voltage on the load constant in presence of variations of the supply voltage is described in the U.S. Pat. No. 6,735,038. The method comprises the driving of a VCM by means of a suitable device coupled with a supply voltage, the generation of commands indicating a voltage desired in output from the device, the comparison of the supply voltage with a reference voltage for generating an error signal, the modification of the commands in function of the error signal and the supplying of the modified commands to the device for generating an output voltage on the basis of the modified commands.